Gels are used due to their specific properties in numerous fields, in particular food, cosmetic and pharmaceutical. A gel consists of at least two components of which one, clearly the majority, corresponds to the liquid solvent and the other is a component that can be classified as a solid dispersed within the solvent. Based on a solution or a dispersion in liquid state, the formation of the gel results from partial aggregation of solid particles. This transformation is called the solution/gel transition.
Compositions for obtaining “physical” gels are well known in the prior art. A physical gel is a macromolecular assembly made up of monomers bonded to each other by low-energy bonds (Van der Waals, hydrogen bonds, polar bonds, etc.). This stability of this assembly is associated with a certain range of physico-chemical conditions (pH, monomer concentration, temperature, solvent quality, ionic force, etc.). Outside this range, the gel becomes a solution again. The solution/gel transition is therefore reversible for physical gels. Thus, the structure of physical gels is highly sensitive to the physico-chemical environment and a very slight change in the quality of the solvent can entirely demolish this structure and thereby produce a gel-solution transition. Conversely, the polymeric association resulting in the gel can be carried out by a slight change in the quality of the solvent.
Gels classified as “chemical” are also known in the prior art. A chemical gel is also a macromolecular assembly, and the monomers it contains are associated by high-energy bonds (covalent). This assembly is therefore very stable. But for all that, while these chemical gels have increased stability, the only way to perform a gel/solution transition consists of destroying the covalent bonds of the polymer. This is why a gel/solution transition of this type is called irreversible.
The chemical gel family corresponds to the enzymatically catalysed gels. This gelling mode is mainly observed in major biological processes. Blood coagulation, cicatrisation, skin formation and the assembly of extracellular matrices are biological processes in which the transition from soluble proteins to gel state is essential, and they share a family of enzymes: transglutaminases (TGases), which are essential in gelling processes. This family of proteins is ubiquitous and can be found equally in prokaryotes and in eukaryotes. Eight different TGases can be found in humans. These enzymes have the property of including amine groups on glutaminyl side chains of proteins. This activity makes it possible to form covalent bonds between the proteins. TGases thus catalyse the polymerisation of the proteins responsible for the formation of biological gelled networks. TGases make it possible to obtain gels from numerous proteins in the food industry, in particular for manufacturing surimi or for hardening numerous meat derivatives (ham, reconstituted food, etc.). Examples of these polymerisable proteins include gelatine, fibrin, gliadin, myosin, globulin (7S and 11S), actin, myoglobin, whey proteins, in particular caseins and lactoglobulin, soy and wheat proteins and, in particular, glutenin, egg white and yolk and, in particular, egg albumin.
One of the most commonly used protein gels is gelatine gel. Gelatine is obtained from collagen, which is a protein with a ubiquitous structure. Collagen can be found in soluble state in the form of monomers or trimers associated in triple helix formation. These triple helices can associate as fibrils which can associate as fibres. The collagen triple helix is unstable at body temperature. Gelatine is obtained by collagen denaturation. Tissue containing collagen is therefore subjected to acid or alkaline treatment, which denatures the collagen triple helix. The possibility of forming fibres is therefore completely lost. Acid treatment results in the formation of type-A gelatine and alkaline treatment produces a type-B gelatine. The gelatine solution therefore consists of isolated collagen chains (collagen monomers). Since gelatine has many uses, it is sometimes necessary to create gelatine gels in conditions where physical gels cannot exist (high temperatures, extreme pH or specific ionic force). To form the network required for the gel, the gelatine chains are then interlaced by covalent bonds and, in particular, by the action of the TGases. The gels thus obtained are chemical gels.
Many fields currently require the use of chemical gels due to their improved stability. However, their “irreversibility” restricts their potential uses in the cosmetics, food or even pharmaceutical industries. Greater control over the mechanical properties of the various gels therefore constitutes an essential issue for increasing their potential.